1. Field of the Invention
The present invention relates in general to radios and, in particular, to a transmitting and receiving system and radio transmitter/receiver apparatus to be utilized within the system, wherein the radio apparatus is capable of transmitting and receiving a transmission signal on two or more radio frequencies, such as found in a system implementing a continuous frequency hopping protocol.
2. Background Art
Radio transmitters and receivers, taking the form of cordless telephones, cellular phones, citizen band radios and the like, are well known in the art. In general, these radios particularly those that receive on multiple channels, utilize a superheterodyne receiver. Superheterodyne (also known as superhet) receivers receive a signal and mix the received signal with a local oscillator signal to, in turn, convert the received signal to a lower intermediate frequency (usually on the order of 10 MHz). Changing the local oscillator frequency tunes the receiver to permit reception of signals on various channels.
The use of a mixer to obtain the intermediate frequency introduces certain problems. The mixer accepts two signals, A and B, the received signal and the local oscillator frequency respectively, and outputs the following signals: A+B, A-B and B-A. The intermediate frequency signal results from the A-B or B-A output. The signal located at A+B will be hopelessly out of range for nominally used frequencies and is typically ignored. Even so, it can be seen that there exist two frequencies for signal A, which result in the same intermediate frequency. One value for signal A, of course, is the desired RF signal. The other is referred to as an image response. It can be seen, from the above equations that the image response's frequency is located twice the intermediate frequency from the desired RF signal's frequency.
There have been a number of approaches to removing or rejecting the image response which accompanies the desired RF signal. One such approach involved the use of tuned RF amplifiers. However, these tuned amplifiers are expensive and difficult to tune. Another approach involves the use of image-reject filters (or mixers). Image-reject filters are also expensive and frequently difficult to design.
Another approach to image rejection involves a method called "direct conversion", wherein the intermediate frequency is set to zero, i.e. the local oscillator frequency is equal to the received transmission frequency. While this approach eliminates the image problem, the processing of the resulting intermediate frequency signal is complex--requiring expensive active filters, which may have inferior noise characteristics. Furthermore, there may be local oscillator frequency reradiation in the channel.
It is, thus, an object of the present invention to provide an inexpensive approach to eliminating image signal concerns in a superhet receiver.
It is a further object of the present invention to provide a compact band-plan for a multiple channel transmitting and receiving system wherein image responses associated with the desired RF signal are positioned such that the desired RF signal is transmitted and received without interference from signals on the image response frequencies.
These and other objects will become apparent in light of the specification, drawings and claims.